Nyiragongo (Democratic Republic of Congo) erupts fluid, fast-moving, foidite lavas. Nearby Nyamuragira frequently erupts less mobile tephrite lavas. Nyamuragira flows rarely threaten urbanized areas, but Nyiragongo flows threaten Goma (pop. ~900,000) and surrounding villages, resulting in fatalities in 1977 and 2002. We report new laboratory measurements of viscosity evolution during cooling and crystallization from both volcanoes by concentric cylinder viscometry. Melt viscosity is ~33 Pa s at the liquidus (~1220°C) for Nyiragongo lavas, similar to Hawaiian basalts. Lavas remain fluid over ~75°C of undercooling (ϕc <0.05) before rapid crystallization forms a crystal network inhibiting flow. Melt viscosity is ~40 Pa s at the liquidus (~1260 C) for Nyamuragira lavas, which remain fluid over ~110 C of undercooling (ϕc <0.11) before the onset of rapid crystallization. Low viscosity allows thin Nyiragongo flows to reach >10 ms-1 even at subliquidus temperatures on low slopes, posing great risk to Goma.